Measuring device and method for determining the position of an electrically conductive test object

ABSTRACT

Measuring device and method for determining the position of an electrically conductive test object ( 1 ) with a noncontacting sensor, in particular an eddy current sensor ( 2 ), wherein the test object ( 1 ) is adapted for linear reciprocal movement in a predetermined direction. The test object ( 1 ) includes a marking ( 6 ), and the sensor is arranged transversely to the direction of movement of the test object ( 1 ) and at a constant distance from the test object in the region of the marking ( 6 ), so that a movement of the test object causes the sensor to produce an at least largely linear signal change over a predetermined measuring range.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of international applicationPCT/DE2004/001705, filed 30 Jul., 2004, and which designates the U.S.The disclosure of the referenced application is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a measuring device and a method fordetermining the position of an electrically conductive test object witha noncontacting sensor, in particular an eddy current sensor, whereinthe test object is adapted for linear reciprocal movement in apredeterminable direction.

Measuring devices and methods of the type under discussion have beenknown from practical operation in a great variety of designs andconfigurations, for example, from DE 101 41 764 A1; U.S. Pat. No.6,762,922; and U.S. Patent Publication No. 2003/0098686. Quitegenerally, such measuring devices and methods are used to definedistances, displacements, or positions relative to any electricallyconductive test objects. Typical applications of noncontactingdisplacement measuring sensors, in particular eddy current sensors,include, for example, the positioning of wafer slices in thesemiconductor production, the detection of vibrations or bearingoscillations, or the monitoring of air gaps in magnetic bearings. Aspecific application of noncontacting displacement sensors, to which theinvention relates in particular, is the determination of test objectsthat linearly reciprocate in one direction, for example, the measurementof immersion depths, or the monitoring of piston strokes or cylinderpositions.

In the case of the measuring devices of the prior art, the sensor isgenerally arranged in the extension of the longitudinal axis of thecomponent being detected, and is aligned parallel to same. Because ofthe linear reciprocal movement of the component, for example, a cylinderpiston, the distance between the end face of the cylinder piston and thesensor increases or decreases in accordance with the actual position ofthe cylinder piston. When an eddy current sensor is used, differentlystrong eddy currents are induced in the piston as a function of thedistance between the piston end face and the sensor, which results in acorresponding output signal on the eddy current sensor. The outputsignal varies linearly with the position of the piston. In the case ofsuch arrangements, the relatively long structural forms are oftenproblematic, which applies in particular when only a limited space isavailable for the measuring device.

Such a situation exists in particular in the case of fuel injectors, asare used, for example, in pump-nozzle injection systems or common-railinjection systems. To monitor and regulate the injection, it has beencommon practice to measure the movement of the injector needle asdescribed above, toward the closing piston. Disadvantageous in thisinstance is, on the one hand, the long structural form of the fuelinjector, which is not needed for the injection itself. This longstructural form is mechanically extremely costly and with that costintensive both in its manufacture and in its maintenance and care.Furthermore, it is disadvantageous that the position measuring of theinjector needle is extremely subjected to tolerances, since the usablemeasuring point is very small, and since it is extremely difficult andpossible only with great adjustment efforts to position the sensor in anexact manner.

It is therefore an object of the present invention to provide ameasuring device and a method for determining the position of anelectrically conductive test object of the initially described type,which make it possible to determine in a very precise manner theposition of a test object with simple constructional means and in acompact structural form.

SUMMARY OF THE INVENTION

In accordance with the invention, the foregoing object is accomplishedby the provision of a measuring device for determining the position ofan electrically conductive test object, which includes a marking on thetest object. The sensor is arranged transversely to the direction ofmovement of the test object and at a constant distance from themeasuring object in the region of the marking, so that a movement of thetest object causes the sensor to produce a signal change that extends atleast largely linearly over a predetermined measuring range.

Furthermore, with respect to a method for determining the position of anelectrically conductive test object, the foregoing object isaccomplished by providing a marking on the test object, and arrangingthe sensor transversely to the direction of movement of the test objectand at a constant distance from the test object in the region of themarking, so that a movement of the test object causes the sensor toproduce a signal change that extends at least largely linearly over apredetermined measuring range.

In accordance with the invention, it has been found that the arrangementof a sensor in the extension of a linearly reciprocating test object isdisadvantageous for many applications because of the resultant,extremely long structural form. Furthermore, it has been found that itis possible to eliminate the found disadvantages in an effective mannerby providing on the test object a marking that influences the fieldlines of the sensor. To this end, the invention provides for arrangingthe sensor transversely to the direction of movement of the test objectat a constant distance from the test object in the region of themarking. During the movement of the test object, the marking thusextends differently deep into the range of the electromagnetic field ofthe sensor, thereby causing on the sensor a signal change that extendsat least largely linearly over a predetermined measuring range.

With respect to a constructionally simple realization, the marking couldessentially comprise a stepped edge, which would permit a differentspacing of the test object from the end face of the sensor on both sidesof the edge. On the one side of the edge the distance from the sensor issmaller than on the other side of the edge, where accordingly greatereddy currents are induced. Consequently, during a movement of the testobject and, with that, of the edge region in front of the sensor, theeddy current induced in the test object changes as a whole, so that as afunction of the position of the test object, the sensor detectsdifferently strong eddy current losses.

In an advantageous manner, the test object could have at least insections thereof a shape similar to a bar or rod. For many applications,however, a cylindrical configuration of the test object would beespecially suitable. The use of the measuring device in an injectionsystem would then permit realizing a constructionally exact guidance ofthe cylindrical test object in the inlet bore with a very small play, sothat the linear movement of the test object occurs always at an exactlyconstant distance from the end face of the sensor.

In a specific realization, it would be possible to provide two steppededges on the test object. In particular, it would be possible toconfigure the two edges such that they define together a recess on thetest object. The recess could be limited to a defined range, when viewedin the circumferential direction of the test object. In this case, thetest object would need to be aligned such that the recess directly facesthe end face of the sensor. As an alternative, it would also be possibleto configure the two edges such that the recess defined by the edges onthe test object is formed in the sense of an annularly extending groove.

For an optimal utilization of the available measuring range, the widthof the recess or groove could approximately correspond to the diameterof the end face of the sensor.

To monitor the time behavior of a fuel injection, preferably inautomobile engines, it would be possible and advantageous to couple thetest object mechanically with a needle of a fuel injector. Bydetermining the position of the fuel injector in this manner, it wouldbe possible to optimize the injection with the use of suitable controlmethods such that the best efficiency or a desired behavior of theengine is achieved in all states of operation. To avoid a costlymechanical connection between the test object and the injector needle,it would be possible and even advantageous to construct the test objectdirectly as an injector needle. In other words, it would be possible toprovide a marking of the above described type directly on the injectorneedle.

In another specific realization, it would be possible to couple the testobject mechanically with a servo-valve of a fuel injector. Likewise inthis instance, it is possible to provide a marking directly on theservo-valve itself.

In a particularly space saving type of construction and to realize anextremely compact structural form, it would be possible to install thesensor in the housing wall of a fuel injector. In this case, one couldprovide that the sensor is displaceable in a channel formed in thehousing wall, so that the end face of the sensor is at leastapproximately flush with the inner wall of the housing. The exactpositioning of the sensor within the channel could be chosen as afunction of the material properties of the test object. In this case,the ferromagnetic properties of the material are of special importance.

With respect to a simple evaluation of the position measurement, itwould be possible to couple the eddy current sensor in a manner knownper se with an electric oscillating circuit. The latter could be adaptedto a basic positioning of the test object. In the case of an edge on thetest object, one could choose the basic positioning such that the edgeof the test object coincides with the center axis of the sensor.

In the case that two edges defining the recess or groove are provided onthe test object, a basic positioning of the sensor is also possibleexactly between the two edges. When the test object moves from its basicposition, the eddy currents induced in the test object will increase. Atthe same time, energy is withdrawn from the oscillating circuit, and thelatter is detuned. Finally, it is possible to provide a correspondingoutput signal by means of adequately known bridge circuits.

In a particularly unique configuration, two sensors are provided, whichare placed in a basic position such that the center axis of one sensorextends over one edge, and the center axis of the second sensor extendsover the other edge. Such an arrangement would permit forming adifferential signal from the two sensors. In the case of a positionchange of the test object, the signal of the one sensor becomes smaller,whereas that of the other sensor simultaneously becomes greater, andvice versa. In this manner, the sensitivity doubles, and likewise thetemperature stability considerably increases, since the temperatureerror in the center of the measuring range will cancel out, i.e. whenboth sensors supply the same signal.

There exist various possibilities of improving and further developingthe teaching of the present invention in an advantageous manner. To thisend, one may refer to the following description of two preferredembodiments of the invention with reference to the drawings. Inconjunction with the description of the preferred embodiments of theinvention with reference to the drawings, also generally preferredimprovements and further developments of the teaching are explained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a first embodiment of a measuringdevice according to the invention for determining the position of anelectrically conductive test object when used in a fuel injector; and

FIG. 2 is a schematic side view of a second embodiment of a measuringdevice according to the invention, when used in a servo-valve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a schematic side view of the first embodiment of ameasuring device according to the invention for determining the positionof an electrically conductive test object 1. The device comprises aneddy current sensor 2 which is arranged transversely to the direction ofmovement of the test object 1. The direction of movement of the testobject 1 is indicated by a double arrow. Specifically, the movement isthe reciprocal movement of an injector needle 3, which mechanicallyconnects to the test object 1. The injector needle 3 forms a part of afuel injector 4 and releases at a defined time interval a nozzle opening5, so that fuel is allowed to enter the combustion chamber at a highrate.

To determine the actual position of the injector needle 3, the testobject 1 is provided in accordance with the invention with a marking 6,which is formed as a groove 7 that annularly surrounds the test object1. In accordance with the invention, an eddy current sensor 2 isoriented transversely to the direction of movement of the test object 1and injector needle 3, and is arranged at a constant distance from thetest object 1 in the region of the groove 7. To this end, a channel 8 isprovided in the wall of the fuel injector 4, into which the eddy currentsensor 2 is inserted. The sensor 2 is positioned such that its end faceis essentially flush with the inner wall of the fuel injector 4, andthus almost contacts the test object 1. The constructionally very exactguidance with little play ensures that the linear movement of theinjector needle always occurs at the same distance from the end face ofthe eddy current sensor 2.

The basic positioning of the sensor 2 and the test object 1 is selectedsuch that the center of the measuring range is exactly indicated, whenone of edges 9 forming the groove 7 (in the embodiment shown in FIG. 1,the upper one of the two edges 9) coincides with the center axis of thesensor 2. The one end of the measuring range corresponds with theclosing position of the injector needle 3, whereas the other end of themeasuring range is reached, when the injector needle 3 has fully opened.When the upper one of the two edges 9 is exactly in the center above thesensor 2 or the sensor coil, i.e., when the basic position is reached,the injector needle 3 will have covered half of its total stroke.

When the injector needle 3 moves from its basic position upward, aconstantly enlarging portion of the groove 7 will move in front of thesensor 2, so that the eddy currents induced in the test object 1decrease. Conversely, when the injector needle 3 moves from its basicposition downward, a constantly enlarging portion of the groove 7 willmove out of the region in front of the sensor 2, so that the inducededdy currents increase. These changes of the induced eddy currents whichare detected by the eddy current sensor 2 as a function of the positionof test object 1 or injector needle 3, are converted into acorresponding output signal, which extends largely linearly over anadequately large measuring range without additional and in generalextremely costly, linearization measures in terms of circuitry orcomputation. This will suffice to determine with great accuracy theposition of edge 9 and, with that, of injector needle 3.

Irrespective of the specific realization of the illustrated fuelinjector 4, the measuring principle can be universally applied to agreat variety of fuel injectors. In particular, it will not matter,whether the test object 1 with groove 7 is mechanically coupled with theinjector needle 3, or whether one uses for measuring an existing edge orgroove on the injector needle 3, or an edge or groove specially providedon the injector needle 3 for measuring the position.

FIG. 2 schematically illustrates a side view of a second embodiment ofthe measuring device according to the invention. The arrangement isbasically similar to that of FIG. 1, with like numerals indicating likecomponents. Unlike FIG. 1, the embodiment of FIG. 2, however, does notdetermine the position of an injector needle, but the position of aservo-valve 11. The servo-valve 11 performs linear movements in thedirection indicated by the double arrow. In accordance with theinvention, the eddy current sensor 2 with a measuring coil 12 isarranged transversely to the direction of movement of the servo-valve 11at a constant distance d. The marking toward which a measurement isperformed, is defined by the edge 9, which is formed by a transitionfrom a valve stem 13 to a valve housing 14. In the basic position, theedge 9 extends exactly along a center axis 15 of the measuring coil 12.The measuring method for monitoring the valve position is analogous tothe method described in connection with FIG. 1.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which theinvention pertains having the benefit of the teachings presented in theforegoing description and the associated drawings. Therefore, it is tobe understood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A measuring device for determining the position of an electricallyconductive test object which is mounted for linear reciprocation,comprising at least two stepped edges on the test object, at least twosensors each arranged transversely to the direction of movement of thetest object and at a constant distance from the test object in theregion of the stepped edges, so that a movement of the test objectcauses each sensor to produce a signal change that extends at the leastlargely linearly over a predetermined measuring range, wherein eachsensor is an eddy current sensor which defines a center axis, andwherein the sensors are arranged in a basic position such that onestepped edge of the test object coincides with the center axis of onesensor, and the other stepped edge of the test object coincides with thecenter axis of the other sensor.
 2. The measuring device of claim 1,wherein the test object has at least in sections thereof a shape similarto a bar or rod.
 3. The measuring device of claim 1, wherein the testobject is cylindrical.
 4. The measuring device of claim 1, wherein thetwo edges form a recess on the test object which faces each sensor. 5.The measuring device of claim 1, wherein the test object is mechanicallycoupled with an injector needle of a fuel injector.
 6. The measuringdevice of claim 1, wherein the test object is constructed as an injectorneedle of a fuel injector.
 7. The measuring device of claim 1, whereinthe test object is mechanically coupled with a servo-valve of a fuelinjector.
 8. The measuring device of claim 1, wherein the test object isconfigured as a servo-valve of a fuel injector.
 9. The measuring deviceof claim 1, wherein each eddy current sensor is coupled with an electricoscillating circuit.
 10. The measuring device of claim 1, wherein thetwo edges form on the test object an annularly extending groove.
 11. Themeasuring device of claim 10, wherein the width of the grooveapproximately corresponds to the diameter of an end face of each sensor.12. The measuring device of claim 1, wherein each sensor is installed ina housing wall of a fuel injector.
 13. The measuring device of claim 12,wherein each sensor has an end face which is at least approximatelyflush with an inner side of the housing wall.